1. Field of the Invention
The present invention relates to an irregular shaped ferrite carrier for a two-component electrophotographic developer used for copying machines, printers, and the like, and to an electrophotographic developer using the ferrite carrier, and relates in detail to an irregular shaped ferrite carrier which has a lowered resistance, a high specific surface area, a low specific gravity and a longer operating life, and to an electrophotographic developer which uses the ferrite carrier and which prevents the toner scattering, has a high image density, and is responsive to the high-speed and full-color imaging.
2. Description of the Related Art
The two-component developer used in electrophotography is constituted of a toner and a carrier, and the carrier is a carrier material which is mixed and agitated with the toner in a developer box, gives the toner a desired charge, carries the charged toner to an electrostatic latent image on a photoreceptor, and forms a toner image. The carrier is, after having formed the toner image, held by a magnet and stays on a development roll, further returned to the developer box, again mixed and agitated with new toner particles, and repeatedly used in a certain period.
The two-component developer, different from a one-component developer, is one in which the carrier agitates the toner particles, imparts a desired chargeability to the toner particles, and has a function of transporting the toner, and which has good controllability in developer design, and is therefore widely used in the fields of full-color machines requiring high-quality images and high-speed machines requiring reliability and durability of image sustainability.
In such two-component electrophotographic developers, an iron powder carrier such as an oxide-filmed iron powder or a resin-coated iron powder has been conventionally used. However, since the iron carrier has a large true specific gravity and then imparts a large stress in developing machines, the life-elongation is difficult.
Then, ferrite carriers such as Cu—Zn ferrite and Ni—Zn ferrite, which have a lower true specific gravity than the iron powder carrier, are used. These ferrite carriers also have many characteristics advantageous over the conventional iron powder carrier in obtaining high-quality images.
As these ferrite carriers, spherical ones are commonly used. However, spherical ferrite carriers have a high resistance, are apt to be insufficient in the developing capability, and can hardly respond to the high-speed imaging. Besides, since they have a small specific surface area, and the low retentiveness of toners, the fogging of image and toner scattering are apt to occur.
Then, for enhancing the developing capability of a spherical ferrite, it is proposed that a resin is coated on the surface of the ferrite core, and a conductive agent is added in the resin to lower the resistance. However, the resin of the resin-coated carrier is apt to exfoliate by use over time, and especially the carrier made to have a lowered resistance by a conductive agent has a large change in the resistance during use period, thereby not being able to achieve a sufficiently longer operating life.
Japanese Patent Laid-Open No. 2000-233930 describes a carrier core composition composed substantially of a spinel phase containing manganese oxide and iron (III) oxide as ferrite components and a certain amount of titanium oxide. Containing titanium oxide in such a manner allows to hold a high conductivity, or a low resistivity, and a saturation magnetization above a certain limit.
However, although a certain low resistance is achieved by containing titanium oxide in the ferrite components, since the shape is not controlled, the conductivity of the ferrite particle of the carrier core material is low, and the toner retention is insufficient, whereby the targeted developing capability cannot be obtained, and troubles such as the fogging of image and toner scattering arise.
On the other hand, use of various irregular shaped ferrite carriers in place of the spherical ferrite carrier is proposed. For example, Japanese Patent Laid-Open No. 2002-116582 describes the use of a carrier of 108 to 1010 Ω·cm in resistivity provided on an irregular shaped ferrite core of 130 or more in shape factor (SF-1) with a coating layer formed by dispersing a conductive powder in a binder resin.
However, with the shape factor (SF-1) specified alone, the conductivity of the magnetic carrier of the carrier core material is not sufficient, and the targeted developing capability cannot be obtained. Moreover, since there is a necessity of using a large amount of conductive powder for enhancing the developing capability, color toners are contaminated, and the image quality degradation is apt to be brought about. Further, when such a large amount of conductive powder is dispersed and contained in the coating layer, the coating layer becomes apt to exfoliate and drop off due to the stress loaded in machine, and thereby the carrier loses its conductivity, which makes it difficult to maintain its favorable characteristics over a long period.
Japanese Patent Laid-Open No. 07-261461 describes a magnetic carrier having an average particle size of 10 to 100 μm nonspherically formed of a magnetite particle having a saturation magnetization of 100 emu/g or more, and describes that it can enhance the image quality and prevent the carrier scattering. Therein, nonspherical shapes include a polyhedron, a multiplanar shape, a scalelike shape, a flat shape and an indeterminate shape.
Although this document proposes that the magnetite is formed into nonspherical particles, whose specific surface area is larger. Since the shape factor and shape distribution are not controlled, the conductivity of the carrier core material as magnetic carrier is low, and the toner retentiveness is not sufficient, so with the higher-speed imaging, a high developing capability is difficult to obtain.
Japanese Patent Laid-Open No. 2002-182434 describes a magnetic carrier in a flat shape whose major axis, minor axis and thickness have a certain relationship and whose easy axis of magnetization is in its plane.
However, when such a magnetic carrier is used, since contact points are scarce, the conductivity is not sufficient, and since the toner retentiveness is nor sufficient, the targeted developing capability cannot be obtained, thus causing troubles such as the fogging of image and toner scattering. The carrier having such a shape has a tendency of being relatively brittle against mechanical impact, and may possibly vary largely its characteristics due to breakage of the carrier particle.
Thus, attempts have not been achieved in which a ferrite carrier is made to have a low resistance, a high specific surface area, a low specific gravity and a longer operating life, and in which when it is rendered into a developer, the toner scattering is prevented, and the developer has a high image density and is responsive to the high-speed and full-color imaging.